The present invention relates to a substrate holding apparatus in which a substrate is held when polished for flattening a surface thereof. The present invention also relates to a polishing apparatus comprising the above-mentioned substrate holding apparatus.
A semiconductor device fabricating process comprises forming a thin film layer on a wafer and forming minute patterns and holes in the layer. This process is repeated until a desired number of circuit layers are formed on the wafer. Therefore, raised and recessed portions are created on or added to the surface of the wafer after formation of each circuit layer. In recent years, semiconductor devices have become increasingly minute and element structures of semiconductor devices have become complicated. Further, there is a tendency to increase the number of circuit layers for logic type devices. As a result, raised and recessed portions on the surface of a semiconductor device increase in number and a difference in height between these portions also increases. This leads to a problem such that during formation of a film on the wafer, an extremely thin film is formed over an undulating area containing the raised and recessed portions on the wafer and breaks in a circuit and an electrical insulation defect between circuit layers are likely to occur, leading to a lowering of product quality and a lowering of yield. Although semiconductor devices can operate normally during an initial period of operation, they are not reliable when used over a long period of time.
Raised and recessed portions on the wafer are also problematic in a lithography process. That is, when an exposure surface of the wafer contains raised and recessed portions, the lenses of an exposure system partially become out of focus, so that formation of minute patterns becomes difficult.
For these reasons, the techniques for surface flattening in fabricating semiconductor devices have been increasingly becoming important. Of various surface flattening techniques, the most important technique is CMP (chemical mechanical polishing), which comprises polishing by using a polishing apparatus, in which while an abrasive liquid containing abrasive particles of silica (SiO2) or the like is supplied onto a polishing surface of a polishing pad, a semiconductor wafer is slidably engaged with the polishing surface.
Conventionally, the polishing apparatus of the above-mentioned type comprises a polishing table including a polishing pad having a polishing surface and a wafer holder for holding a semiconductor wafer. The wafer holder is adapted to hold a semiconductor wafer and press the wafer against the polishing table under a predetermined pressure. The wafer holder and the polishing table are moved relative to each other so that the semiconductor wafer is slidably engaged with the polishing surface, to thereby polish the wafer to a flat and mirror-finished surface.
In the above-mentioned polishing apparatus, when a relative pressure generated between the semiconductor wafer and the polishing surface of the polishing pad is not uniform over an entire surface of the wafer, insufficient or excessive polishing is likely to occur, depending on the pressure acting on each part of the wafer. Therefore, in order to apply a uniform pressure to an entire surface of the wafer, an elastic membrane made of rubber is provided on the wafer holder on a surface thereof for holding a wafer, and a fluid pressure such as air pressure is applied to a back surface of the elastic membrane. In this case, a circumferential edge of the wafer surface corresponds to a boundary between a contact portion and a non-contact portion of the wafer relative to the polishing surface. Since the polishing pad is elastic, the pressure applied to a portion around the circumferential edge of the wafer surface becomes non-uniform, so that only the circumferential edge of the wafer is polished in an excessive amount, and the wafer is caused to have a “dull” edge.
As a countermeasure, it has been proposed to use a wafer holder in which a guide ring or retainer ring for holding an outer circumferential edge of the wafer presses the polishing surface at a position outside the wafer. In this wafer holder, the retainer ring is pressed against the polishing surface under fluid pressure such as air pressure.
FIG. 14 is a schematic illustration of a wafer holder of the above-mentioned type, in which a fluid pressure is applied to a wafer so as to press the wafer against a polishing surface, and the fluid pressure is also applied to a retainer ring so as to press the retainer ring against the polishing surface.
As shown in FIG. 14, a wafer holder 50 comprises: a wafer holder body 51 defining an inner space; a wafer pressurizing mechanism 52 contained in the inner space of the wafer holder body 51 and adapted to press a semiconductor wafer W against a polishing surface 61 of a polishing table 60; a retainer ring 53 provided so that it is vertically movable relative to the wafer holder body 51 and adapted to hold an outer circumferential edge of the wafer W; and a retainer ring pressurizing mechanism 54 for pressing the retainer ring 53 against the polishing surface 61.
The wafer pressurizing mechanism 52, although not shown in detail, comprises an elastic membrane member which is made of an elastic material such as rubber and is connected to the wafer holder body 51. A pressurized fluid such as pressurized air is supplied to the inside of the elastic membrane member so that the wafer W is pressed against the polishing surface 61 under fluid pressure. The retainer ring pressurizing mechanism 54, although not shown in detail, also comprises an elastic membrane member which is made of an elastic material such as rubber and is connected to the wafer holder body 51. A pressurized fluid such as pressurized air is supplied to the inside of the elastic membrane member so that the retainer ring 53 is pressed against the polishing surface 61 under fluid pressure. The wafer holder body 51 is connected to a drive shaft 55 and the drive shaft 55 is adapted to be vertically moved by a lifting mechanism such as an air cylinder.
The lifting mechanism such as an air cylinder connected to the drive shaft 55 is operated so as to move the wafer holder body 51 as a whole to a position close to the polishing table 60. While the wafer W is held in proximity to the polishing surface 61, the pressurized fluid is supplied under a predetermined pressure to the wafer pressurizing mechanism 52, to thereby press the wafer W against the polishing surface 61 of the polishing table 60. The pressure applied to the wafer W during polishing is adjusted to a desired value by adjusting the pressure of the pressurized fluid supplied to the wafer pressurizing mechanism 52. On the other hand, the pressurized fluid is supplied under a predetermined pressure to the retainer ring pressurizing mechanism 54, to thereby press the retainer ring 53 against the polishing surface 61 of the polishing table 60.
Since the wafer W is pressed against the polishing surface 61 by using a fluid pressure, it is possible to obtain a uniform pressure distribution across an entire surface of the wafer W from the center to the circumferential edge thereof. This enables uniform polishing of the entire surface of the wafer W. Further, during polishing, a pressure substantially equal to that applied to the wafer W is applied to the retainer ring 53 through the retainer ring pressurizing mechanism 54, so that the polishing surface of the polishing pad outside the wafer W is pressed under a pressure substantially equal to that of the wafer W. Therefore, a uniform pressure distribution can be obtained continuously across an area from the center of the wafer W to an outer circumferential portion of the retainer ring 53 outside the wafer W. Therefore, excessive or insufficient polishing at the circumferential edge of the wafer W can be prevented.
In the above-mentioned conventional wafer holder in which both the wafer and the retainer ring are pressed under fluid pressure, the retainer ring is capable of moving in either a vertical (or perpendicular) direction or a lateral (or radial) direction relative to the wafer holder body. That is, the retainer ring is capable of moving independently of the wafer holder body. Movement of the retainer ring affects uniformity in the polishing of an outer circumferential portion of the wafer surface. Although vertical movement of the retainer ring is necessary for polishing, lateral movement of the retainer ring is unnecessary. Rather, lateral movement of the retainer ring is undesirable because it varies the distance between the retainer ring and the circumferential edge of the wafer surface and impairs uniformity and stability in the polishing of the outer circumferential portion of the wafer surface.
Further, in the conventional wafer holder, since the surface of the wafer holder for holding a wafer is covered with the elastic membrane, it is required to form, for example, a suction cup-like configuration in the elastic membrane so as to hold a wafer during transfer thereof. When a wafer is held by the elastic membrane having a suction cup-like configuration, warpage or deformation of the wafer occurs. Due to warpage of the wafer, the wafer can be broken during transfer thereof or a device structure formed on the wafer can be damaged. Further, since the wafer is held by indirect contact with the wafer holder through the elastic membrane, defects in holding of the wafer are likely to occur during transfer of the wafer, leading to a lowering of operating rate of the wafer holder and a lowering of yield of wafers.
Further, in chemical mechanical polishing (CMP) utilizing an elastic polishing pad and an abrasive liquid (slurry), the following problem arises. That is, when a wafer surface having raised and recessed portions is polished, the raised portions are polished in preference to the recessed portions during an initial period of polishing, but after the raised portions are polished by a certain amount, the recessed portions are also gradually subjected to polishing (as well as the raised portions). Therefore, the difference in height between the raised portions and the recessed portions cannot be easily reduced. That is, because polishing is conducted by using a relatively soft, elastic polishing pad and a slurry type abrasive liquid containing a large amount of free abrasive particles, chemical mechanical polishing is effected on not only the raised portions, but also the recessed portions of the wafer surface. Further, the effect of polishing varies, depending on the density of raised and recessed portions.
Therefore, an attempt has been made with respect to polishing by using a polishing surface comprising fixed abrasive particles such as cerium oxide (CeO2), which are bound by using a binder such as a phenol resin. In this polishing, the polishing surface is hard as compared to the polishing pad conventionally used in chemical mechanical polishing, so that the raised portions are polished in preference to the recessed portions and the recessed portions are unlikely to be polished. Therefore, absolute flatness of the wafer can be easily obtained.
However, a wafer holder suitable for a hard polishing surface comprising fixed abrasive particles has not been developed. Generally, a conventional wafer holder for the hard polishing surface comprises a rigid wafer holder body and an elastic backing pad provided on the rigid wafer holder body adapted to be engaged with a wafer to be held by the wafer holder. Although the elastic backing pad can absorb shocks on the wafer, it is difficult for the elastic backing pad to take care of undulations on the hard polishing surface, whereby the undulations are transferred to and affects the wafer surface to be polished.